Implementation of a High Reliability Photoelectric Switch

Photoelectric switches are non-contact measurement sensors with a wide detection range. They are widely used in many measurement and control systems such as counting, distance measurement, and stroke control. However, the output signal of the photoelectric sensor is related to the intensity of the light-emitting tube, the distance between the light-emitting tube and the receiving tube, and external interference light. Therefore, when using each photoelectric switch, the sensitivity of the receiving circuit must be adjusted first to ensure that the photoelectric switch works in the best state.

This article introduces a set of photoelectric switch circuits, which have the advantages of anti-interference from external light, no need for manual adjustment of sensitivity, stable and reliable operation, etc. It can be used in reflective or through-beam photoelectric switches.

This circuit consists of two parts: light-emitting circuit and photoelectric receiving circuit.

1. Light-emitting circuit

Figure 1 shows a LED driver circuit with high power output, which has the ability to emit 15kHz modulated light.

The first level 4001 is a single pulse generator, which can output detection pulses by manual keystroke for fault maintenance.

In order to stabilize the frequency of the modulated optical signal, a frequency divider CD4060 is used in the circuit. It has an external crystal oscillator and a multi-level frequency divider inside. For a 1M crystal oscillator, after six frequency divisions of 4060, a square wave with a stable frequency of 151625kHz can be obtained. Then, through the current amplification of the power field effect tube, hundreds of light-emitting diodes can be driven to emit light at the same time.

Figure 1 High-power LED driver

2 Photoelectric switch receiving circuit with automatic gain control

Figure 2 shows the detection circuit of the phototransistor. The infrared phototransistor T2 has a base lead pin. Therefore, its sensitivity can be controlled. The photocurrent output by its emitter is inverted by the amplifier T3 and then fed back to the base of the phototransistor. Since there is a low-pass filter composed of R13 and C11 in the feedback loop, this feedback is a negative feedback for the DC operating point, which also controls the AC voltage gain. This is the automatic gain control (AGC) circuit.

Figure 2 Photoelectric receiving circuit

When the input light signal is strong, the collector signal of T3 tends to become stronger, causing the DC operating point voltage of the base of the phototransistor T2 to drop, thereby reducing the AC output of both T2 and T3. Therefore, this negative feedback system will make the AC output signal of T3 almost independent of the light intensity obtained by T2 in a large range. It can be seen that when the input light of the receiving circuit changes, the output signal of T3 does not change much. That is to say, the change in the distance between the light source and the phototransistor has little effect on the output signal of T3 in a large range. Only when the light source is fully blocked, the output signal of T3 will change significantly.

U11 in Figure 2 forms a second-order bandpass filter that can filter out signals other than 151625kHz. The gain of this stage is 1, the Q value is 5, and the cutoff frequency, center frequency and filtering characteristics are easy to adjust. Diode D1 and capacitor C14 form a detection circuit that can demodulate the envelope signal from the 151625kHz signal. The above two circuits can filter out the effects of interference sources such as sunlight, fluorescent lamps, and incandescent lamps.

The output circuit in Figure 2 consists of amplifier stage U12 and Schmidt circuit U13. The hysteresis of U13 can eliminate the critical jitter of the photoelectric switch and avoid the false flipping of the photoelectric switch. Finally, the output state of the photoelectric switch is displayed by the light-emitting diode D2.

3 Experimental data

Table 1 shows the experimental data. The light-emitting tube and the receiving tube face each other, and there is no focusing optical element between them. The main parameters of the whole machine are tested with or without light shielding. It can be seen from the table that the relative distance between the surfaces of the two tubes is within the range of 0.15 to 1.70 mm, and it can work normally without adjusting the sensitivity.

Table 1 Photoelectric switch complete machine experimental data

When a 40W incandescent lamp is used to illuminate the receiving tube at a distance of 100mm, the receiving tube can still work normally, which means that the receiving tube will not be saturated and has strong anti-interference ability. If the working distance is to be increased, an optical lens should be added; when the light irradiation distance is more than 2m, a semiconductor laser diode can be used as a modulated light source.

4 Conclusion

The advanced photoelectric switch circuit introduced in this article has the following characteristics:

① Due to negative feedback, the receiving phototransistor is not easy to saturate.

As long as the tube is not saturated, external light interference can be suppressed.

② Due to the use of modulation and demodulation circuits, it can work reliably under various magnetic, electrical and optical interferences.

③ The Schmitt circuit can debounce the switch.

④ Since the circuit uses integrated circuits as much as possible, especially the application of high-power FETs, the circuit introduced in this article is more suitable for array photoelectric switch applications.

⑤ Due to the use of AGC circuit, the receiving sensitivity almost does not need to be adjusted.

The circuit introduced in this article is used to perform joint precise counting on an automatic production line of 100 tiny products and achieves good results.